1. Field of the Invention
The invention relates to a process for the preparation of oleum having a concentration of 10 to 45% by weight of SO3 and sulphuric acid having a concentration of 94 to 100% by weight by joint combustion of sulphur with atmospheric oxygen and if appropriate SO2-containing and (NO)x-containing gases from the thermal cleavage of sulphuric acid, conversion of the SO2-containing gases to SO3-containing gases in the presence of vanadium catalysts and absorption of the SO3-containing gases to produce oleum and/or sulphuric acid.
2. Brief Description of the Prior Art
The literature discloses differing processes for the preparation of oleum and sulphuric acid. In the thermal processes (sulphur combustion using atmospheric oxygen, metal sulphide roasting, sulphuric acid cleavage, sulphate processes), in addition to sulphur-dioxide-containing gases, there are also formed nitrogen oxides, (NO)x, which are produced directly from nitrogen and oxygen above 1 000° C. Additional (NO)x is introduced by the starting products themselves. Under the combustion conditions, (NO)x is without exception NO. The (NO)x is absorbed, as are also the sulphur-trioxide-containing gases which are formed from the SO2-containing gases in the presence of the vanadium catalyst, so that the sulphuric acid formed or the oleum is contaminated with absorbed (NO)x in the form of nitrosylsulphuric acid. It is assumed that (NO)x is converted to nitrosylsulphuric acid in the following manner:NO+SO3→NO2+SO2  Equation 1NO+NO2+H2SO4→NOHSO4+H2O  Equation 2
Nitrosylsulphuric acid, which is analytically detectable by hydrolysis with water to give HNO2, is a powerful corrosive agent for steel and chrome-nickel stainless steels and leads to considerable surface erosion on apparatuses and piping. To reduce the nitrosylsulphuric acid content, therefore, according to a known proposal, 40% strength dihydrazine sulphate solution is introduced.
DE-A-4 002 465 describes a process by means of which up to 95% by weight of the total nitrogen oxides can be removed. DE-A-4 002 465 relates to a process for the continuous preparation of oleum at a concentration of 10 to 45% by weight SO3 and/or H2SO4 by combustion of sulphur with atmospheric oxygen by the principle of superstoichiometric and substoichiometric combustion, cooling the resultant sulphur-dioxide-containing gases to 390° C. to 480° C., catalytic conversion of these gases to sulphur-trioxide-containing gases in the presence of a vanadium-containing catalyst using the principle of single or double contact catalysis, absorption of the sulphur-trioxide-containing gases after cooling, if appropriate removal of liquids from the gases after absorption and energy production, the combustion of sulphur with atmospheric oxygen being carried out with the addition of dry SO2-containing gases which contain up to 5 000 ppm of (NO)x, preferably less than 2 000 ppm of (NO)x, calculated as NO. The SO2-containing dry gases, which can contain up to 5 000 ppm of (NO)x, calculated as NO, used are cleavage gases from the thermal cleavage of waste sulphuric acids, with these cleavage gases being able to contain 5 to 10% by volume of O2, 5 to 8% by volume of SO2, <200 ppm of CO, <1 000 ppm of (NO)x, <50 ppm of hydrocarbon compounds and 82 to 90% by volume of H2O, N2 and CO2. Using these processes, the dry SO2-containing gases can be introduced directly into the combustion chamber in which sulphur is burnt with oxygen, or can be introduced into the combustion chamber after mixing with combustion air. It is important that the SO2-containing gases (nitrous SO2-containing gases) are burnt jointly with sulphur. In a temperature variant, sulphur is burnt at temperatures between 500° C. and 1 000° C. (measured at the outlet of the combustion chamber upstream of gas cooling), chiefly between 700° C. and 950° C.
The known process has the disadvantage that a breakdown of up to 95% by weight of (NO)x occurs. However, the residual amounts of (NO)x, as may be seen from the examples, lead to concentrations of >25 mg of NO/m3 (S.T.P.). In practice this means that in the preparation of oleum of high concentrations of 30 to 45% by weight, as is required for the preparation of pure sulphur trioxide and oleum 65%, virtually all of the (NO)x was absorbed in the form of nitrosylsulphuric acid and led to a high accumulation in the oleum cycles of the oleum distillation. Not until the concentration is below 27% free SO3 does the degree of absorption of (NO)x decrease noticeably.
It is an object of the present invention, therefore, in the existing process, to decrease the (NO)x content considerably below 95% to contents of <5 mg of NO/m3 (S.T.P.).
Surprisingly, this object is achieved by the inventive process. The advantages of the inventive process are the reduction of the nitrosylsulphuric acid content, the reduction of the maintenance costs which are caused by corrosion, and the complete avoidance of the use of dihydrazine sulphate solution within the meaning of the objective according to section 37 of the German Dangerous Substance Act of replacing carcinogenic working materials.